With the objective of new insight into integrative mechanisms of intermediate levels, the approach of comparing favorable species in lower vertebrates and invertebrates is used. Methods are mainly electrophysiological, at the single and multiple unit level. Experiments fall into four groups. (1) Central analysis of sensory input. This includes acousticolateral especially electroreceptors and others, and ontogenetic development and plasticity of connectivity in such systems. (2) Control of output in selected systems. (3) Transactions in extended arrays of neurons. This includes the comparative study of EEG, evoked and similar potentials. (4) Comparison of neural differences between behaviorally distinct species, e.g. sharks and rays, porpoises and seals, sloths and non-slow mammals, groups of electric fish.